Many residences and commercial buildings in temperate climates have both heating and air conditioning systems. The heating systems are generally used during the winter and the air conditioning during the summer, but frequently during the spring and fall both systems are utilized during different periods of the day. Thermostats previously provided for these dual function systems typically included means for storing a first temperature set point for the heating system which may be manually adjusted to a level of say, 68 degrees, and separate means for storing a set point for the cooling system which may be adjusted to a higher temperature, such as 74 degrees. If the two set points are adjusted too close to one another the system may oscillate between heating and cooling modes because the temperature may overshoot the heating set point during the heating mode and undershoot the cooling set point during the cooling mode. This undesirable oscillation may also occur as a result of thermal exchange between adjacent zones in a multi-zone heating system. If the set points are set too widely apart, to minimize the possibility of oscillation, the comfort of the occupants, who would prefer a single constant temperature, is diminished.
These problems are exacerbated when multitemperature programmable thermostats of the type disclosed in the U.S. Pat. No. 4,172,555 are employed since the maximum temperatures that may be programmed during a heating cycle must be limited to avoid unintentional energization of the cooling system and vice-versa.